Abstract
AbstractPhaseolus vulgaris L., common bean (Pv), is adapted to subtropical and well-watered area; Phaseolus acutifolius A. Gray, tepary bean (Pa) is adapted to hot arid climates of deserts in the US. and Mexico. Field observations and physiological studies suggest that Pa has more heat and drought tolerance than Pv, and could be a good donor to Pv. Therefore it has attracted attention of many investigators. However, in search of the reasons why Pa is more tolerance than Pv, their differences in photosynthesis, one of the most important physiological processes of plants, still has not been well documented. In this study, we studied the different responses of Pa and Pv in photosynthesis, photoinhibition and the metabolism of starch under various stress conditions.The photosynthesis of Pa and Pv, including light and dark reactions were monitored chlorophyll fluorescence. Enzymatic assay was used to measure the content of sucrose and starch.Under heat stress, the PSΠ activity, electron transport rate, and the ability of CO2 fixation of Pv became lower, and it also developed a problem in transport of starch and sucrose. So they accumulated in cells, which in turn retarded growth of Pv. However, Pa had greater tolerance to heat stress than Pv in photosynthesis. The results even suggest that high temperature is more suitable for Pa than room temperature.Under drought stress, the dark reactions of Pa and Pv were both inhibited, but Pa was inhibited to a lesser extent. Notably the electron transport rate of Pa was accelerated to enhance starch synthesis, which resulted in higher concentration of sucrose and thus higher water retention capacity. The sucrose concentration of Pv also became higher, but regulated worse. Its starch synthesis was decelerated and almost exhausted. As for photosynthesis, we found that at the similar water content, there are no difference in the extent of photosynthesis inhibition between Pa and Pv. It suggests that a greater drought tolerance of Pa is a result of it has higher water retention capacity.Under heat and drought double stresses, the PSΠ activity, electron transport rate, and the ability of CO2 fixation of Pv became lower, but those of Pa were even a little higher. In the experiment with leaves, we found that photosynthesis of both Pa and Pv were well protected under the double stresses. But Pa obviously had better ability to retain its PSΠ activity than Pv under heat and extreme drought condition. Under the double stresses, the starch content of Pa and Pv were greatly enhanced by three to four times. It might result from the loss of hexose transporter function, which caused starch to accumulate in chloroplasts. Of course this conjecture requires more studies.Under any conditions, normal or stress, Pa is more tolerance to photoinhibition than Pv did. The photoinhibition in Pv was more serious when under drought stress than under heat stress, and the effect of heat and drought double stresses was more profound than that of only one kind of stresses. But these stresses had hardly any effect on the photoinhibition of Pa . In summary, this study clearly demonstrates that the photosynthesis system of Pa performs better than Pv under heat, drought or heat and drought double stresses, including its PSΠ activity, electron transport rate, and the ability of CO2 fixation. Moreover, the regulation of photosynthesis produce transport of Pa is better than Pv, so is the ability of retaining water content and keeping healthy physiological functions.摘要……………………………………………………..1Abstract…………………………………………………4前言……………………………………………………..6實驗材料與方法………………………………………19結果……………………………………………………26討論……………………………………………………36圖目……………………………………………………43參考文獻………………………………………………74